VU meter woes

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  • altavek
    Member
    • Jun 2013
    • 12
    • USA

    #1

    VU meter woes

    So I decided to finally fixed the burnt out bulb in my Tascam M-3700's VU meters. I happened to have some nice white LEDs from some dead solar lights and thought that would be a nice change from the aged yellow of the tube bulbs that were in there. Dropped a 1k resistor in front and everything meters out fine. However, the meter that had the burnt out bulb still doesn't light up. I double checked the bulb itself and it is in fact popped. Quadruple checked everything on the PCB and everything is good. It lights up when I put the diode setting on my MM to it from various locations on the board but still no light when I plug in the PCB. Popped the hood on the master section of the board itself and checked all the connects are good and seated to no avail.

    Ideas?
    Attached Files
  • budm
    Badcaps Legend
    • Feb 2010
    • 40746
    • USA

    #2
    Re: VU meter woes

    Is the old lamp being fed by AC or DC, and what is the Voltage feeding the old lamp?
    Never stop learning
    Basic LCD TV and Monitor troubleshooting guides.
    http://www.badcaps.net/forum/showthr...956#post305956

    Voltage Regulator (LDO) testing:
    http://www.badcaps.net/forum/showthr...999#post300999

    Inverter testing using old CFL:
    http://www.badcaps.net/forum/showthr...er+testing+cfl

    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
    http://s807.photobucket.com/user/budm/library/

    TV Factory reset codes listing:
    http://www.badcaps.net/forum/showthread.php?t=24809

    Comment

    • redwire
      Badcaps Legend
      • Dec 2010
      • 3906
      • Canada

      #3
      Re: VU meter woes

      Fuse-style VU meter lamps are fed from AC, around 6VAC (DC gives shorter lifetime and you need bigger filter caps $). So I would expect a white LED to fail. They can't take any reverse voltage. I'd put a 1N914 across them.

      Comment

      • altavek
        Member
        • Jun 2013
        • 12
        • USA

        #4
        Re: VU meter woes

        Well there was a + and a - on the board so I had assumed it DC. And I metered the bulbs at about 7VDC but if they are AC, I'll look in to the semi conductors. Thanks!

        Comment

        • budm
          Badcaps Legend
          • Feb 2010
          • 40746
          • USA

          #5
          Re: VU meter woes

          If it is DC, max you current with White LED will be about 4mA (@7V, 3V drop for LED vF, 4V drops on 1K = 4mA) through the LED, which will barely conduct. Also do you have Positive o the Anode of the LED? You should drive the LED with 15~20mA range to get the brightness as spec by the LED manufacturer or what ever the spec is for this LED. Most TASCAM use AC for lamp. popular lamps are rated at 6v, 8v, and 12V.
          Last edited by budm; 09-06-2013, 02:33 PM.
          Never stop learning
          Basic LCD TV and Monitor troubleshooting guides.
          http://www.badcaps.net/forum/showthr...956#post305956

          Voltage Regulator (LDO) testing:
          http://www.badcaps.net/forum/showthr...999#post300999

          Inverter testing using old CFL:
          http://www.badcaps.net/forum/showthr...er+testing+cfl

          Tear down pictures : Hit the ">" Show Albums and stories" on the left side
          http://s807.photobucket.com/user/budm/library/

          TV Factory reset codes listing:
          http://www.badcaps.net/forum/showthread.php?t=24809

          Comment

          • altavek
            Member
            • Jun 2013
            • 12
            • USA

            #6
            Re: VU meter woes

            When I looked at the PCB again, the +/- was at the terminal posts for the needle not the light. Everything meters correct so has to be AC. For 10¢ a piece, worth it to try the SCs.

            Comment

            • budm
              Badcaps Legend
              • Feb 2010
              • 40746
              • USA

              #7
              Re: VU meter woes

              Unless you use full-wave rectifier (will flicker at 120Hz) or half-wave (will flicker at 60Hz) you will need small filter cap to reduce the flicker.
              Never stop learning
              Basic LCD TV and Monitor troubleshooting guides.
              http://www.badcaps.net/forum/showthr...956#post305956

              Voltage Regulator (LDO) testing:
              http://www.badcaps.net/forum/showthr...999#post300999

              Inverter testing using old CFL:
              http://www.badcaps.net/forum/showthr...er+testing+cfl

              Tear down pictures : Hit the ">" Show Albums and stories" on the left side
              http://s807.photobucket.com/user/budm/library/

              TV Factory reset codes listing:
              http://www.badcaps.net/forum/showthread.php?t=24809

              Comment

              • momaka
                master hoarder
                • May 2008
                • 12170
                • Bulgaria

                #8
                Re: VU meter woes

                Originally posted by budm
                If it is DC, max you current with White LED will be about 4mA (@7V, 3V drop for LED vF, 4V drops on 1K = 4mA) through the LED, which will barely conduct.
                Most modern white LEDs are fairly bright even at 1 mA. 4 to 6 mA should be plenty. Depending on the LED, 15 to 20 mA may be too much, and if not cooled very well will make the LED have a very short life.

                Also, technically speaking, LEDs can take reverse voltage, as long as you don't exceed the reverse current. However, the reverse current is usually very small and easy to exceed - hence why having an LED in reverse is not a good idea.

                Comment

                • budm
                  Badcaps Legend
                  • Feb 2010
                  • 40746
                  • USA

                  #9
                  Re: VU meter woes

                  The reverse voltage break down is only about 5V, I will not run 7V reverse voltage on it. I can never run at 1mA to get usable brightness, I run then at 20mA (that is only 3.3v x 0.02A = .066W) in the products I designed with good reliability history, typical white LED is rated at 30mA easily. He needs to check out his circuit connection and the spec of the LED he is using.
                  Never stop learning
                  Basic LCD TV and Monitor troubleshooting guides.
                  http://www.badcaps.net/forum/showthr...956#post305956

                  Voltage Regulator (LDO) testing:
                  http://www.badcaps.net/forum/showthr...999#post300999

                  Inverter testing using old CFL:
                  http://www.badcaps.net/forum/showthr...er+testing+cfl

                  Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                  http://s807.photobucket.com/user/budm/library/

                  TV Factory reset codes listing:
                  http://www.badcaps.net/forum/showthread.php?t=24809

                  Comment

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